The present invention relates to head adapter extensions for use in the reactor vessel of a nuclear pressurized water reactor.
Pressurized water reactors which are currently in use include a reactor vessel containing the reactor core and control rods which are movable relative to the core inorder to control its reactivity. Each control rod is suspended from a drive rod that extends through the reactor vessel head to a drive mechanism located above the head.
In order to seal the vessel head at the location of such drive rod, there is provided a cylindrical housing which extends downwardly from the drive mechanism and through a bore in the head to a location within the upper portion of the reactor vessel. The portion of the housing which extends through the vessel head includes a head adaptaer secured in the associated bore, by welding, in order to form a seal between the housing and the vessel head.
The housing further includes a head adapter, or housing, extension which opens, at its lower extremity, to the interior of the vessel and which is generally terminated, at its lower end, by a guide funnel that opens downwardly toward the interior of the vessel and that serves to guide a drive rod into the housing when the vessel head is installed on the vessel body. When such a reactor vessel is assembled, an annular passage which communicates with the interior of the reactor vessel will be created between the drive rod and the associated housing which passes through the vessel head.
In a proposed new pressurized water reactor model, there will also be employed displacer rods which will be movable relative to the reactor core and which will be supported by a drive rod passing upwardly through the vessel head to a drive mechanism. Here again, each drive rod is to be surrounded by a housing having the form described above.
The portion of each housing which passes through the vessel head is made of inconel in order to suitably fasten the vessel head, which is currently made of carbon steel clad with standard stainless steel, to the housing of the drive mechanism itself, which is made of standard stainless steel. BEcause of the expenses involved in the manufacture of inconel parts, this is achieved by forming the part of the housing which is connected to the vessel head as a short head penetration adapter made of Inconel and by then welding a stainless steel pipe to the upper end of the adapter, the upper end of the stainless steel pipe then extending, and being connected to, a further housing enclosing latch components of the drive mechanism.
The head adapter and pipe are joined together by a full penetration weld and it is necessary to take account of the possibility of failure of that weld, which would result in the leakage of liquid out of the reactor vessel via the annular passage between the housing and its associated drive rod.
As protection against the adverse consequences of such a weld failure, or of a leak due to other causes in the housing surrounding the drive rod, it is desirable to limit th cross-sectional area of the flow passage between the interior of the vessel and the source of such a leak to a value of no greater than approximately 5 cm.sup.2.
To achieve this, it has previously been proposed to insert a cylindrical tube, known as a thermal sleeve, in the housing to surround the drive rod and to provide orifices between the thermal sleeve and the head adapter such that the cross-sectional area of the remaining flow passage from the interior of the reactor vessel to the region of the drive mechanism was restricted to the above-cited value. In this structure, an extension rod must be provided above the vessel head.
The use of a such a thermal sleeve, however, is accompanied by a number of disadvantages, among which are tht it incrases the total number of components required and the total manufacturing cost.